Existing film code readers are designed to read a 23-bit, two track DX bar code which is imaged on the edge of 35 mm film during film manufacture. The DX code provides information about the manufactured film, including type, speed, and manufacture. The DX code information is conventionally used in calculating exposure time when the film is printed. This DX code has a 0.015" minimum bar width, a clock and data track, a start and end sentinel, and a parity bit. All these features are used to verify and assure the validity of the DX code when read. A reader for DX code is optimized to read the code, provided that there are sharp transitions in density or contrast between the film background and DX code. This contrast is visible as the darkness of the code with respect to the background.
A problem with the DX code is that it is limited to film information existing at the time of manufacture. Additional information is often desired as to parameters designated by the user of the film for printing negatives of developed film that can only be available after manufacture. Additional bar codes may be exposed on the film while in a camera to overcome this problem. These codes have bar widths three or four times greater than DX codes, and hence are called fat bits. However, due to the uncontrolled environment of a camera, as compared to the environment during film manufacture, the density, integrity, size and shape of the fat bits vary greatly making reading difficult. Reading of fat bits is further made difficult because any imperfections as may be caused by debris, scratches, edge fog, chemical stains during the film developing process, etc. . . can adversely effect exposed fat bits, or create false fat bits. The above conditions make reading fat bits difficult for conventional DX code readers.